Habitable Planets, Some "serious" planetary systems Options

[karolp]

And by serious I mean those with multiple planets, relatively nearby (within 100 or 50 lyrs) and... potentially harbouring some Earth analogues as well. With about 200 extrasolar planetary systems, we are finally nearing some profound conclusions on how planets around other Suns might be arranged and where other Earths would fit in there. I'd love to see some coverage and discussion on those here. All in all, we might exercise such babies like COROT and Kepler on them within a few years.

To start with, consider those two:

55 Cancri - or "the one with 4 planets, including a jupiter at 5 AU and possibly an earth at 1 AU



and

HD 69830 - or "the one with 3 neptunes, an asteroid belt and an earth - maybe..."



As for 55Cnc, there is some disagreement whether the parent star is slightly more or slightly less massive than the Sun. Please compare those sources for that:

Exoplanets.org

Planet Quest

The Extrasolar Planets Encyclopedia

The way those systems are gradually discovered is like focusing an image in an eyepiece. They gradually arrive at more accurate values for the number and orbits of planets by examining wobles (observed as Doppler shifts) of the parent star measured over ever longer periods of time. You can actually hear Geoff Marcy as he is sitting in the control room of Keck and gives directions to the guy operatng the scope (just go to the 12th minute of that program):

(May 2004) - Geoff Marcy discovering exoplanets live ;-)

You can also here some news directly from him here:

(September 2004) - More of Geoff Marcy

And do not forget Chris Lovis:

...also known as "the French guy who discovered those 3 neptunes"

[Greg Hullender]

Looking at the power-law relationship between the planets of 55 Cancri, I get a 0.9977 r^2 if I assume there are two undiscovered planets (or asteroid belts) between "c" and "d". The data here are thin though.

[karolp]

TWO asteroid belts? Would they survive the tugging and tossing by neighbouring planets? 55 Cnc is significantly older then our Solar System...

[Greg Hullender]

Well, actually I was thinking more along the lines of two planets or one planet and one asteroid belt. It might be hard to distinguish two adjacent belts from one BIG belt. :-)

--Greg

[Toymaker]

There is also this usefull table(sadly it can't be updated)
http://www.extrasolar.net/starlisttour.asp?StarcatID=normal

[Jyril]

The Extrasolar Planets Encyclopedia is reliable and always up-to-date.

[Toymaker]

The Extrasolar Planets Encyclopedia is reliable and always up-to-date.

Yes, I know, but to amatours like me, the graphic interface on extrasolar.net is very attractive and clear with its basic informations on mass, probable temperature etc.
I hope somebody will make similiar site or the current one will be adopted by someone.

[Jyril]

I like the Extrasolar Visions, but browse it with caution: It is very speculation-based. Much of the derived data presented there is not reliable.

[karolp]

Making updated databases of all exoplanets (currenly over 200) is crucial alright, but I wish to focus on the most promising ones. I believe there are only a few with 3 planets and more. I keep searching old space.com news and such. I think we may start regarding them similar to our own Sol system, discovering new bodies, refining their orbits and speculating upon what may be going on at their surfaces and in their atmospheres. There are really only a few such systems, about 10. Here goes another one:

Gliese 876 - or "the one with a scorched Earth's cousin and two Jupiters"

[ugordan]

What I'd like to know is how can they come out so certain of a planet's mass using the Doppler shift detection? As far as I understand it, they can measure the star's wobble, but they cannot know the inclination of the planet's orbit to us.

Isn't the mass inferred from doppler shift dependant on the tangent of the inclination or something like that?
So a massive planet whose orbital plane is seen face-on will exhibit a weaker wobble than a small planet whose orbit is edge-on. Are they assuming a statistical inclination of 45 degrees or so and calculate the mass from thereabouts?

If so, saying "we've discovered a 16 Earth-mass planet" seems pretty misleading. Furthermore, they use these statistically most probable masses to do dynamic/evolution simulations of the system. How accurate can that be then?

[karolp]

So a massive planet whose orbital plane is seen face-on will exhibit a weaker wobble than a small planet whose orbit is edge-on.

That is exactly the case. They always get a MINIMUM mass. But if they are lucky, they may catch a transiting planet and hence derive the inclination and true mass value. Nonetheless, the planetary systems with multiple planets and at least some characteristics determined or models run are still at least as interesting as our good ol' system around Sol.

[Jyril]

The "mass" that is given by a star's radial velocity is actually M / sin(i), where i is the inclination of the planet.

i = 90° (edge on, transiting planet): the minimum mass = true mass
i = 60°: mass = 1.2 x minimum
i = 30°: mass = 2 x minimum
i = 10°: mass = 5.8 x minimum
i = 1°: mass = 57 x minimum (1 M_Jupiter = massive brown dwarf)
i = 0.5°: mass = 115 x minimum (1 M_Jupiter = low mass red dwarf)

So, based on this the true mass of an extrasolar planet should usually be less than twice the minimum mass. Planets about the mass of Jupiter or less form about a half of the planets detected so far, so most likely almost all planet candidates are true planets (if we don't count pulsations mimicking planets etc.)

In addition, there are far less brown dwarfs orbiting Sun-like stars there should be, if they were anywhere as common as red dwarfs or giant planets. They should be much easily discovered, but only a few are found. The paucity is called the "brown dwarf desert" and may be related to star birth processes (brown dwarfs may be ejected stellar embryos etc.)

[Jyril]

Interestingly, only a few days before the first post in this thread a fourth planet orbiting the star µ Arae was announced. The planet has an orbit very similar to that of Earth and has a mass of half Jupiter's mass.

The first planet discovered orbits at the distance of 1.5 AU and has a mass of 1.67 M_Jupiter. The second planet orbits much further at 4.17 AU and is three times as massive as Jupiter. The third planet is the most interesting, the first "hot Neptune", being only ten times as massive as Earth and orbits at a distance of 0.09 AU. The system is also interesting in the respect that most orbits are relatively circular (at least compared to most extrasolar planets). However, it is known that the current parameters cannot be right; the system is stable less than 100 million years. Especially the orbit of the outermost planet is not very well defined.

[karolp]

Yeap, that would be the only 4-planet system apart from 55 Cnc that we know so far. But 3-planet systems are also not that common, which makes it easy to focus our attention on them and do some comparison and speculation.

Those with 3 planets start with Upsilon Andromedae or The First Known to Harbor 3 Planets:



So, to sum things up, those with 3 planets are:

Gliese 876 at 15 lyrs
HD 69830 at 41 lyrs (3 neptunes)
Ups And at 44 lyrs
HD 37124 at 107 lyrs

And those with 4 planets:

55 Cnc at 44 lyrs
HD 160691 at 49 lyrs (mu Arae)

[Guest_AlexBlackwell_*]

For those with access to American Scientist, there's an interesting article (cover story, in fact) by Gregory P. Laughlin, entitled "Extrasolar Planetary Systems," in the September-October 2006 issue.